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Basis of Presentation (Textual) (Details) |  |  |  | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 22, 2018 USD ($) a | Mar. 15, 2017 USD ($) ft² | Jul. 01, 2016 USD ($) a | Jun. 30, 2018 USD ($) a Parcel | Jun. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Jun. 30, 2018 USD ($) a Parcel | Jun. 30, 2017 USD ($) | Mar. 16, 2018 a | Dec. 31, 2017 USD ($) | Apr. 28, 2017 a | Jan. 30, 2017 a
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Prepaid leasehold interests in land held for development | [1] |  |  |  | $ 40,177,000 |  |  | $ 40,177,000 |  |  | $ 0 |  |
Income tax provision |  |  |  |  | 20,000 | $ (339,000) |  | 158,000 | $ (239,000) |  |  |  |
Increase in operating cash flows |  |  |  |  |  |  |  | 3,656,000 | 2,262,000 |  |  |  |
Lease agreement variable income |  |  |  |  |  |  |  | 0 |  |  |  |  |
Third party management contracts assets outstanding |  |  |  |  | 0 |  |  | 0 |  |  |  |  |
Third party management contracts liabilities outstanding |  |  |  |  | 0 |  |  | 0 |  |  |  |  |
Net gain on sale of undepreciated real estate |  |  |  |  | 2,837,000 | 0 |  | 2,859,000 | 0 |  |  |  |
Proceeds from the sale of properties |  |  |  |  |  |  |  | 16,771,000 | 102,083,000 |  |  |  |
Assets held for sale, net |  |  |  |  | 4,254,000 |  |  | 4,254,000 |  |  | $ 392,000 |  |
Gain (Loss) on Sale |  |  |  |  | (35,000) | 1,088,000 |  | (35,000) | 8,411,000 |  |  |  |
Philadelphia Marine Center (Marine Piers) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  | 181,900 |  |  |  |  |  |  |  |  |  |
Sales Price |  |  | $ 21,400,000 |  |  |  |  |  |  |  |  |  |
Payment by cash on purchases |  |  | 12,000,000 |  |  |  |  |  |  |  |  |  |
Proceeds from the sale of properties |  |  | 11,200,000 |  |  |  |  |  |  |  |  |  |
Balance payment on purchases |  |  | $ 9,400,000 |  |  |  |  |  |  |  |  |  |
Date of second installment payment |  |  | Jan. 31, 2020 |  |  |  |  |  |  |  |  |  |
Assets held for sale, net |  |  | $ 4,700,000 |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale |  |  | 6,500,000 |  |  |  |  |  |  |  |  |  |
Deferred gain on sale |  |  | $ 9,400,000 |  |  |  |  |  |  |  |  |  |
ASU 2016-18 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash |  |  |  |  | $ 900,000 | 33,200,000 |  | $ 900,000 | 33,200,000 |  |  |  |
Increase in operating cash flows |  |  |  |  |  |  |  |  | 500,000 |  |  |  |
Increase in investing cash flows |  |  |  |  |  |  |  |  | 32,000,000 |  |  |  |
General and Administrative Expenses [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax provision |  |  |  |  |  | 300,000 |  |  | 200,000 |  |  |  |
Net Income Tax Provision [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Reclassification of net income tax provision out of general and administrative expenses into net income tax provision |  |  |  |  |  | $ 300,000 |  |  | $ 200,000 |  |  |  |
3001-3003 JFK Boulevard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Lease agreement term |  | 99 years |  |  |  |  |  |  |  |  |  |  |
Area of developed parcels of land (in acres) | a |  | 1 |  |  |  |  |  |  |  |  |  |  |
Prepaid leasehold interests in land held for development |  | $ 24,600,000 |  |  |  |  |  |  |  |  |  |  |
Garza Land Acquisition [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of developed parcels of land (in acres) | a |  |  |  | 34.6 |  |  |  |  |  |  |  |  |
Gross purchase value of assets |  |  |  | $ 20,600,000 |  |  |  |  |  |  |  |  |
Number of Parcels | Parcel |  |  |  |  | 3 |  |  | 3 |  |  |  |  |
Garza Land Acquisition [Member] | Parcel One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  |  |  |  |  |  |  | 8.4 |
Garza Land Acquisition [Member] | Parcel Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  |  |  |  |  |  |  |  | 1.7
Garza Land Acquisition [Member] | Parcel Three [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  |  |  |  |  |  | 6.6 |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of developed parcels of land (in acres) | a |  |  |  | 34.6 |  |  |  |  |  |  |  |  |
Gross purchase value of assets |  |  |  | $ 20,600,000 |  |  |  |  |  |  |  |  |
Number of Parcels | Parcel |  |  |  |  | 3 |  |  | 3 |  |  |  |  |
Net gain on sale of undepreciated real estate |  |  |  |  | $ 2,800,000 |  |  |  |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  | 8.4 |  |  | 8.4 |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel One [Member] | Third Party Developers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  | Jan. 30, 2017 |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Two [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  | 1.7 |  |  | 1.7 |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Two [Member] | Third Party Developers [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  | Apr. 28, 2017 |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Three [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Area of land sold | a |  |  |  |  | 6.6 |  |  | 6.6 |  |  |  |  |
Garza Land Acquisition [Member] | Garza Land Sale [Member] | Parcel Three [Member] | Third Party [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of parcels sold |  |  |  |  |  |  |  | Mar. 16, 2018 |  |  |  |  |
Other Assets [Member] | 3001-3003 JFK Boulevard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Prepaid leasehold interests in land held for development |  | $ 24,900,000 |  |  |  |  |  |  |  |  |  |  |
Impairment Charges [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis Of Presentation [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Out-of-period error immaterial in current year |  |  |  |  |  |  | $ 1,200,000 |  |  |  |  |  |
[1] | As of June 30, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information

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